Aluminum alloy



R. H. BROWN ALUMINUM ALLOY Feb. 1, 1938.

Filed May 25, 1936 5 no 0 Q @C B 5 .Q

Percent Magnesium (x) INVENTOR. ROBERT H. BROWN.

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Patented Feb. 1, 1938 UNITED STATES m FFICE AL ALLOY Application May 25, 1936, Serial No. 81,599

10 Claims.

This invention relates to aluminum base alloys containing from about to 7 per cent of magnesium, and it is more particularly concerned with improving the resistance to stress cracking 5 of these alloys when they are in the cold worked condition.

Aluminum-magnesium alloys are distinguished from many other aluminum base alloys in respect to their lower specific gravity and generallybetter resistance to corrosion. They have, however, suffered from the drawback of being diflicult to work, and it has only been through the application of a method of hot workingsuch as described in U. S. Patent No. 1,926,057, that the production of wrought products on a commercial scale has been possible. The final fabricating operations in the manufacture of wrought articles are generally performed at ordinary temperatures, with the result that the finished product is in a cold worked condition. The cold working done in this manner increases the strength and hardness of the alloys but reduces the elongation. Such an increase in strength and hardness is very desirable for certain types of service.

Cold worked aluminum-magnesium a l l o y s which contain from about 5 to 7 per cent magnesium are sometimes characterized by a peculiar cracking or structural failure when subjected to high internal or external stresses in the pres- 3 ence of corrosive solutions'or gases. This type of failure which is herein designated stress cracking is to be distinguished from a common kind of corrosion wherein the surface of the metal is pitted. Both stress cracking and pitting attack may occur in the same piece of metal, and it is possible by suitable means to suppress one without affecting the other. Stress cracking appears only when the metal is in a strained condition whereas ordinary corrosion occurs independently of any stress. The cold worked aluminum-magnesium alloys do not always fail in this manner, and, in fact, under ordinary conditions, stress cracking seldom appears. However, since it does occur under severe conditions, and because it is more deleterious than ordinary corrosive attack, its elimination ordiminution is highly desirable and important. My invention is directed towards preventing and inhibiting the formation of stress cracks in the aforesaid type of alloy without substantially impairing the physical properties.

I have discovered that the addition of small amounts of zinc within certain limits to aluminum-magnesium alloys containing from 5 to 7 per cent magnesium will substantially reduce the (c1. rte-s2) stress cracking tendencies in the cold worked alloy and yet not materially diminish the resistance to ordinary corrosive attack. of zinc required to achieve this result is closely defined, and varies with the concentration of magnesium. For the range of 5 to 6 per cent magnesium, from about 0.25 to 1 per cent of zinc is effective in improving the resistance to stress cracking without adverse effect upon other properties of the alloy. In alloys containing from 6 to 7 per cent magnesium, the maximum amount of zinc which it is permissible to use to overcome the stress cracking tendency of the alloy without adversely affecting the resistance to ordinary corrosive attack decreases approximately linearly from 1 to 0.75 per cent and the minimum amount required to counteract stress cracking increases linearly from 0.25 to 0.75 per cent.

While there is a very definite improvement in .the resistance to stress cracking in alloys containing zinc within the foregoing proportions, I have found that the optimum condition of resistance to both stress cracking and ordinary corrosive attack is present in alloys having the following compositions: 5.5 per cent magnesium, 0.5 per cent zinc; 6 per cent magnesium, 1 per cent zinc; 6.5 per cent magnesium, 0.9 per cent zinc; and 7 per cent magnesium, 0.75 per cent zinc. If these compositions are plotted on a graph as in Figure 1, with the abscissa, X, representing the magnesium concentration and the ordinate, Y, the zinc content, and a line be drawn through these points, the'curve so drawn will have the following equation.

Y= 15520.931 1.9166X +58.373X -708.809X

This equation is only useful between the values of 5 and 7 per cent magnesium. An approximation of the relationship between the magnesium and zinc may be obtained by interpolating between the compositions mentioned above. Such an approximation is satisfactory for commercial operations because of allowances that are usually made for deviations in composition.

The maximum and minimum values for zinc in alloys containing between 5 and 7 per cent The amount' magnesium are represented by the broken lines in- Figure 1. While these limits are constant at 1.0 and 0.25 per cent, respectively, between 5 and 6 per cent magnesium, it will be observed, that between 6 and 7 per cent magnesium they change with respect to each other and finally converge at 0.75 per cent zinc in an alloy containing 7 per cent magnesium. Although the curve representing the alloys having optimum properties does not exactly conform to a straight line between 6 and 7 per cent magnesium it does closely approximate a straight line, and comes .within the.

operating limits of alloy composition on commercial scale production. For the purpose of defining a maximum limit for the zinc, the curve and straight line are considered to be substantially identical. The limits on composition of the alloy are of course subject to the deviations encountered in commercial practice and hence alloys need not precisely conform to the compositions mentioned hereinabove to come within the scope of my invention and show improvement in resistance to stress cracking.

Inasmuch as aluminum-magnesium alloys are inherently more resistant to corrosion than many other aluminum base alloys it is eminently desirable that this property should be preserved as far as possible. I have determined that the addition of zinc within the range specified above does not injure the resistance of the alloy to ordinary attack, but that if larger proportions of zinc are employed the latter property suffers. I have also found that at least 0.25 per cent zinc is needed to inhibit stress cracking.

My invention is limited to cold worked alloys containing from 5 to'7 per cent magnesium since there is an almost total absence of stresscracking in alloys containing less than 5 per cent of this element and the difliculties involved in cold working alloys on a commercial .scale become very great, where the magnesium content exceeds 7 per cent. Furthermore, the zinc addition is relatively ineffective in inhibiting stress cracking in cold worked alloys if more than 7 per cent magnesium is present. a

The surprising eifect of zinc in inhibiting stress cracking in cold worked aluminum base alloys containing 5 to 7 per cent. magnesium is shown by the following examples. Sheets from two alloys composed, respectively, of aluminum and 6.19 per cent magnesium; and aluminum, 6.1 per cent magnesium and 1 per cent zinc, were annealed at 650 F., cold rolled with a reduction of 50 per cent in cross sectional area, and aged at 212 F. for 4 hours. The last thermal treatment was designed to stabilize the alloys and render them uniform in structure. A reduction in cross section of 50 per cent is a severe working of this type of alloy and it therefore provides a critical test of any method for diminishing stress cracking. Specimens cut from these sheets were mounted in afixture which was adjusted to apply a stress equal to A of the yield strength of each alloy. "The yield strength in each case had been previously determined from blank specimens cut from the same sheets. The mounted specimens were then immersed in a 6 per cent aqueous solution of sodium chloride and made anodic with an external potential of 0.9 volt by connecting them with an external source of electric current. Under these severe conditions, specimens of the alloy without zinc showed se- Vere intergl'amllar ng and broke in 72 hours while the specimens containing zinc did not break within this time nor did they show any cracks even upon microscopic examination. Under similar test conditions only a small percentage of the specimens of an alloy containing about 6 under ordinary conditions for the alloys containing zinc.

Another test covering a more extended period of time showed the following results. In this test, strips from the cold worked sheets were bent in an arc and alternately'immersed in and raised from a 3.5 per cent aqueous solution of sea salt, a complete cycle occupying 60 minutes. The specimens from an alloy containing 6.2 per cent magnesium broke in 3 months while the specimens from an alloy containing 6 per cent magnesium and -1 per cent zinc did not break in 20 months.

The addition of zinc, furthermore, does not impair the physical properties of the cold rolled alloy. The three alloys mentioned above had the following properties in the cold rolled condition.

Tensile Yi Percent Amy th Strength lbs/sq. in. lbs./s in elongation containing from 5 to 7 per cent magnesium the cold working is the chief factor contributing to susceptibility to stress cracking. There is no critical amount of cold work required to produce this condition except that a reduction of approximately 50 per cent renders the alloy most vulnerable to this type of failure. The tendency to stress crack diminishes with more or less than this amount of cold work but it still remains as long as any permanent set is produced in the metal below the recrystallization temperature. The manner of cold working is unimportant since the result is the same in any case. Hot working the alloy above the recrystallization temperature, however, does not produce the proper condition for stress cracking and hence my invention is only applicable to alloys deformed below the recrystallization temperature, a term which is well understood by those skilled in the art.

In order to improve certain properties such as hardness and grain size which are related to the grain structure of the aluminum-magnesiumzinc alloys described above, without at the same time impairing the fundamental characteristics of these alloys, one or more of the metals manganese, chromium, molybdenum, tungsten, titanium and zirconium may be incorporated with the aluminum magnesium zinc composition. The foregoing metals are alike in respect to increasing the hardness and reducing the grain size of the base alloy, and hence for the purpose of nrv invention they are considered as constituting a group of equivalent substances which may be used either separately or in combination. The

. amounts of the individual metals which will effect the desired improvement are as follows: 0.05 to 1 per cent manganese, 0.05 to 0.5 per cent chromium, and 0.05 to 0.25 per cent each 01' molybdenum, titanium, tungsten, vanadium or zirconium. The total amount of these metals may vary between 0.05 and 1.5 per cent, and where two or more are employed,'at least 0.05 per cent of each one should be present.

The zinc may be added to the alloy in any manner practiced in the art.

Any good commercial grade of aluminum may be used in making the alloys. However, I prefer to employ aluminum of a purity of 99.6 per cent or higher since alloys made 'from such metal are more resistant to ordinary corrosion.

I claim:

1. A cold worked aluminum base alloy con- .taining from to 7 per cent magnesium and a relatively small amount 01' zinc dependent upon the magnesium content, the proportion of zinc being between 0.25 and 1 per cent for a magnesium content of 5 to 6 per cent, and with linearly converging maximum and minimum limits of 1 to 0.75 per cent and 0.25 to 0.75 per cent zinc, respectively, with an increasing concentration of magnesium of from 6 to 7 per cent, said alloy being characterized by substantial freedom from stress cracking.

2. A cold worked aluminum base alloy composed of aluminum, magnesium and zinc, said alloy containing from 5 to 7 per cent magnesium and up to 1 per cent zinc, the proportion of zinc being determined by the magnesium content of the alloy, there being between 0.25 and 1 per cent zinc in alloys containing 5 to 6 per cent magnesium, with a constantly diminishing maximum of 1.0 to 0.75 per cent, and an increasing minimum 01 0.25 to 0.75 per cent zinc as the H magnesium content increases from 6 to 7 per cent.

3. A cold worked aluminum base alloy containing from 5 to 7 per cent magnesium and up to 1 per cent zinc, the proportion oi. zinc being determined by the magnesium content of the alloy, there being between 0.25 and 1 per cent zinc in alloys containing 5 to 6 per cent magnesium, with a constantly diminishing maximum 01' 1.0 to 0.75 per cent, and an increasing minimum of 0.25 to 0.75 per cent zinc as the magnesium content increases from 6 to 7 per cent, and a total oi from 0.05 to 1.5 per cent oi at least one ,metal selected from the group of hardening elements composed of 0.05 to 1 per cent manganese, 0.05 to' 0.5 per cent chromium, and 0.05 to 0.25 per cent each of molybdenum, titanium, tungsten, vanadium; and zirconium, the balance of the alloy being aluminum.

4. A cold worked aluminum base alloy composed of aluminum, .5 to 6 per cent magnesium and 0.25 to l per cent zinc. Y

5. A cold worked aluminum base alloy composed of aluminum, 6 to '7 per cent magnesium, and .a linearly diminishing maximum of 1.0 to 0.75 per cent, and a linearly increasing minimum of 0.25 to 0.75 per cent zinc with an increasing magnesium content of from 6 to 7 per cent.

6. A cold worked aluminum base alloy containing from 5 to 7 per cent magnesium and a. relatively small amount of zinc dependent upon the magnesium content of the alloy, the proportion of zinc being not over that defined by the equation:

Y=15520.931--1.9166MH-58.3'13X'708.809X +4288.4756X=-- 12925.0392X wherein X represents the magnesium and Y the zinc contents of the alloy, the balance of the alloy being aluminum. v v

8. A cold worked aluminumbase alloy composed 0! about 5.5 per cent magnesium and about 0.5 per cent zinc, the balance being aluminum.

9. A cold worked aluminum base alloy composed of about 6.5 per cent magnesium and about 0.9 percent zinc, the balance being aluminum.

10. A cold worked aluminum base alloy composed 01 about 7 per cent magnesium and about 0.75 per cent zinc, the balance being aluminum.

ROBERT E. BROWN. 

